The VENUS coastal ocean network includes a variety of instruments associated with acoustics. Some are active, in that they transmit and receive signals to probe the ocean, while others are passive and listen to the sounds in the ocean. The Hydrophone arrays are passive listening devices, that have several sensing elements, or hydrophones, connected to a central processing unit for analog to digital conversion. A hydrophone is similar to a microphone, but is designed specifically to work underwater. Unlike light, sound propagates efficiently in the ocean, and sounds can often be heard many kilometers from their source.
There are three primarily types of sources for sound in the ocean: marine mammals, human or anthropogenic sources, and environmental sources, such as rain, thunder, wind, and geophysical (i.e. earthquakes). To monitor and record these sounds, we deploy small arrays of multiple broadband (wide frequency range) hydrophones. Although located near the bottom, the hydrophones pick-up sounds generated throughout the water column, and from great distances. Because sound reflects and refracts in complicated ways as it travels in the stratified ocean, the sounds we hear are often modified and combined in strange ways, producing complex and ghostly effects. In addition to listening to the sound, we may visualize the content of a sound clip by plotting a time history of spectra (see Ocean Processes). These plots reveal how the frequency content varies over time (X-axis), with higher frequencies increasing along the Y-axis and more intense sound energy represented by warmer (redder) colours.
Why only near real time: The present VENUS hydrophone arrays have a central processor subsea that transmits data to a dedicated shore-station computer for logging. A requirement from the Navy is that all our hydrophone data pass through a specially designed switch before it is committed to disk or buffered. This is so they (the Navy), who have control over the switch, can divert the signals if they are conducting exercises or deem there are Navy ships/resources in the area which they do not want us recording/broadcasting. Our hydrophone logging software is quite configurable, and one of the settings is the duration of the files. We process the audio in blocks, so there is a trade-off between small and fast, with many files, and slightly longer and fewer files. Through experience, we have found that 5 minutes files when recording at 44kHz is a reasonable trade-off of the various issues. When we conduct calibrations at 120kHz sample rates, we log one minute files. While the logging software has these active files open (recording), they are not accessible. Once closed (after 5 minutes), they are almost immediately fetched by an FTP daemon and verified in our secure archive back here at UVic. This takes another few minutes. These raw files are highly compressed in a custom format that is not readably distributable or usable by any external software. Once archived, we process these raw (file extension *.hyd) files, doing some QA/QC checks and producing spectrogram plots and MP3 audio files. Although not ideal for research purposes, MP3 is a good broadcast format since the files are small and most computers can easily play them for listening. These products are then copied over to our web server, and the links refreshed. This whole process (from logging to posting) can take as little as ten minutes. If there is latency or heavy loads on any of our systems, then it can take longer (20-30 minutes).
VENUS provides the hydrophone data in a number of formats, including the original raw data in our own custom digital file format (*.hyd), spectrogram plots, and audio files (i.e. MP3). For the MP3 files, the audio level has been normalized so that for each clip, the peak trace (wave form) alternating between -1 and 1 has a magnitude of 0.7. This normalization compresses the original dynamic range. The effect is to reduce loud sounds and amplify soft sounds, preventing a listener from having to constantly adjust the speaker volume while listening to multiple clips. An un-normalized WAV file can also be generated. Contact us for further information on our hydrophone data.
icListen LF Hydrophone Data
The icListen LF (low frequency) hydrophone, presently undergoing technology demonstration testing on VENUS, is a low frequency hydrophone manufactured by Instruments Concepts (www.instrumentconcepts.com) in Nova Scotia. The nominal frequency range of this hydrophone is 0.1 Hz to 1.6 kHz. This is a 24 bit hydrophone, sampled at 4 kHz, with a mid band sensitivity of -27.3 dB re counts/uPa. This hydrophone is currently deployed next to the #3 broadband hydrophone on the SoG East hydrophone array. Calibration data from 0.02Hz to 1600 Hz is available on request.
Each spectrogram is 5 minutes long. The FFT window size is 4000 samples (1 s) with a 50% overlap. The hydrophone sensitivity and the Hann window attenuation compensation are applied on a 1 Hz bin basis.
